410 MANUAL OF ZOOLOGY. 



looked upon as most characteristic of the Gasteropods, the 

 shell being composed of a number of whorls passing obliquely 

 round a central axis or "columella," having the embryonic shell 

 or "nucleus" at its apex, and having the mouth or "aperture" 

 of the shell placed at the extremity of the last and largest of 

 the whorls, termed the "body-whorl" (fig. 216). The lines or 



Fig. 216. Anterior and posterior views of Cassis cancellata, a spiral Gasteropod. a 

 " Spire," placed at the posterior end of the shell ; b " Mouth," placed at the anterior 

 end of the shell ; c Inner or columellar lip ; d Outer lip ; e Notch for the passage of 

 a respiratory siphon. 



grooves formed by the junction of the whorls are termed the 

 "sutures," and the whorls above the body-whorl constitute the 

 " spire " of the shell. The axis of the shell (columella) round 

 which the whorls are coiled is usually solid, when the shell is 

 said to be " imperforate ; " but it is sometimes hollow, when 

 the shell is said to be " perforated," and the aperture of the 

 axis near the mouth of the shell is called the "umbilicus." 

 The margin of the "aperture" of the shell is termed the 

 "peristome," or "peritreme," and is composed of an outer and 

 inner lip, of which the former is often expanded or fringed 

 with spines. When these expansions or fringes are periodi- 

 cally formed, the place of the mouth of the shell at different 

 stages of its growth is marked by ridges or rows of spines, 

 which cross the whorls, and are called "varices." In most of 

 the phytophagous Gasteropods (Holostomata) the aperture of 

 the shell (fig. 217) is unbrokenly round or "entire," but in the 

 carnivorous forms (Siphonostomata) it is notched, or produced 

 into a canal (fig. 218). Often there are two of these canals, 

 an anterior and a posterior, but they do not necessarily indi- 

 cate the nature of the food, as their function is to protect the 

 respiratory siphons. The animal withdraws into its shell by 

 a retractor muscle, which passes into the foot, or is attached 





